1. Field of the Invention
The invention relates to an image heating apparatus for heating an image on a recording material. As such image heating apparatus, mention can be made of a fixing apparatus for fixing an unfixed image on a recording material, or a gloss improving apparatus for heating an image fixed on a recording material to thereby improve the degree of glossiness of the image.
2. Description of the Related Art
Heretofore, in an image forming apparatus such as a printer or a copying machine using an electrophotographic printing material, there has been provided a fixing apparatus for fixing a toner image on a sheet.
In Japanese Patent Application Laid-Open No. H02-157880, as such a fixing apparatus, there is proposed one using a belt. An example of the conventional belt fixing apparatus will be shown below.
The conventional belt fixing apparatus 100 shown in FIGS. 6, 7 and 8 of the accompanying drawings is constituted by a fixing unit 100A having a fixing roller 101 provided with a halogen heater (not shown) therein, and a belt conveying apparatus 133. The belt conveying apparatus 133 is constituted by a pressure unit 100B, a release cam 108, a deviation control driving portion 115, etc. The pressure unit 100B has a suspension roller 102 and a tension roller 103 as suspension members for circulating a suspended pressure belt 104.
The opposite ends of the rotary shaft 102a of the suspension roller 102 are rotatably supported on a pair of supporting metal plates 105 by antifriction bearings (not shown). The opposite ends of the rotary shaft 103c of the tension roller 103 are also rotatably supported on a pair of slide metal plates 106 by antifriction bearings (not shown). The slide metal plates 106 are slidably provided on the supporting metal plates 105. The slide metal plates 106 are biased away from the supporting metal plates 105 by tension springs 107. Therefore, the suspension roller 103 is biased away from the tension roller 104 to thereby give a tensile force to the pressure belt 104.
Also, when a pair of release cams 108 (one of which is not shown) are rotated by a pulse motor (not shown) the pressure unit 100B is adapted to be rotated about a pivot shaft 109 and is moved toward and away from the fixing roller 101 as a nip forming member.
As shown in FIG. 8, a driving force from a drive source 110 is inputted to the pressure belt 104 of the pressure unit 100B through an idler gear 111 on the pivot shaft 109. Therefore, the pressure unit 100B has the driving force inputted thereto whether it is in a pressed state or in a spaced-apart state relative to the fixing roller 101.
Such a fixing apparatus 100 presses the pressure belt 104 of the pressure unit 100B against the fixing roller 101 of the fixing unit 100A to thereby form a fixing nip portion between the fixing roller 101 and the pressure belt 104. The fixing nip portion, when a recording material S bearing an unfixed toner image thereon is conveyed thereto, nips and heats the sheet as the recording material together with the toner image. The recording material S is pressurized and heated, whereby the toner image thereon is fixed.
As described above, the fixing apparatus 100 using the belt enables the nip width in the conveyance direction of the recording material S to be wider than in a fixing apparatus (not shown) using a pair of rollers comprising a fixing roller and a pressure roller, and can obtain high fixing performance.
The endless belt of such a belt conveying apparatus, when it is circulated, may sometime be deviated in the width direction thereof crossing the circulation direction of the belt. Usually, the deviation of the belt is continued until as shown in FIG. 9 of the accompanying drawings, the pressure belt 104 comes into contact with a deviation restricting flange 112 provided on an end portion of a roller and the deviation thereof is restricted. In this case, however, the slide edge portion of the pressure belt 104 and the flange 112 may sometimes rub against each other and the side edge portion may be shaved thereby.
The main cause of the belt being circulated while being deviated is largely the inclination difference between the suspension members such as the suspension roller 102 and the tension roller 103 which are not disposed in parallelism to each other. In other words, by changing the inclination of the suspension members, it is possible to control the deviation direction of the belt.
So, the deviation of the belt can be adjusted if the deviation information of the belt is obtained from deviation detecting sensors 113a and 113b disposed in opposed relationship with the side edges 104a and 104b of the pressure belt 104 (see FIG. 8) to thereby change the inclination of the tension roller 103. The changing of the inclination of the tension roller 103 is accomplished by moving only one end portion of the tension roller 103 up and down by a deviation control arm 114 rotated in a vertical direction by a deviation control driving portion 115.
The deviation control of the belt will now be specifically described with reference to FIGS. 10, 11 and 12 of the accompanying drawings. When the pressure belt 104 is rotated in the direction indicated by the arrow A and becomes deviated in the direction indicated by the arrow B, the deviation detecting sensor 113a detects the side edge 104a of the pressure belt 104 to thereby detect the deviation of the pressure belt 104. On the basis of the result of the detection, a controlling portion (not shown) controls the operation of the deviation control driving portion 115 and rotates the deviation control arm 114 by a predetermined value in a clockwise direction as viewed in FIG. 7. Thereupon, the end portion 103a on this side of the tension roller 103 rises in the direction indicated by the arrow C as shown in FIG. 11, and the pressure belt 104 deviates in the direction indicated by the arrow D.
The inner part side refers to the side on which the drive source 110 is disposed. Also, this side refers to the side on which the deviation control driving portion 115 is disposed.
The end portion 103b on the inner part side of the tension roller 103 is tiltably fixed to the slide metal plate 106 disposed on the inner part side. The end portion 103a on this side of the tension roller 103 extends through the longitudinal slot 106a (see FIG. 7) of the slide metal plate 106 and is vertically movable. Accordingly, the tension roller 103 is designed such that the end portion 103a on this side is vertically moved with the end portion 103b on the inner part side as a fulcrum and the tension roller 103 is inclined as a whole.
When as shown in FIG. 12, the pressure belt 104 deviates in a direction opposite to the direction indicated by the arrow B, the deviation detecting sensor 113b (see FIG. 8) detects the other side edge 104a of the pressure belt 104. On the basis of the result of the detection, the controlling portion (not shown) controls the operation of the deviation control driving portion 115 and rotates the deviation control arm 114 by a predetermined value in a counter-clockwise direction as viewed in FIG. 7. Thereupon, the end portion 103a on this side of the tension roller 103 lowers in the direction indicated by the arrow E as shown in FIG. 12, and the pressure belt 104 deviates in the direction indicated by the arrow F.
As described above, the belt conveying apparatus 133 continues the operation of vertically moving the end portion 103a on this side of the tension roller 103 while the pressure belt 104 is circulated, whereby it can control the deviation of the pressure belt and circulate the pressure belt 104 to a substantially predetermined position.
In the above-described belt fixing apparatus, however, with the termination of the fixing operation, such an inconvenience as will be described below has occurred when the pressure belt 104 has been spaced apart from the fixing roller 101. That is, when standby rotation is performed with the pressure belt spaced apart from the fixing roller, disorder has occurred to the deviation control of the belt.
When in FIG. 7, the pressure belt 104 is brought into pressure contact with the fixing roller 101, the release cam 108 is rotated to thereby move the supporting metal plate 105 and the slide metal plate 106 toward the fixing roller 101. At this time, the suspension roller 102 rises integrally with the supporting metal plate 105. However, the end portion 103b on the inner part side of the tension roller 103 rises integrally with the slide metal plate 106 on the inner part side, but the end portion 103a on this side is received by the deviation control arm 114 and does not rise. Accordingly, the tension roller 103 is changed in its inclined state relative to the suspension roller 102. Therefore, there has occurred the necessity of operating the deviation control arm 114 to thereby readjust the inclination of the tension roller 103 to its original inclination.
Also, when as shown in FIG. 13 of the accompanying drawings, the tension roller 103 is received with the deviation control arm 114 lowered, the end portion 103a on this side of the tension roller 103 will not rise even if the release cam 108 is rotated to thereby move the two metal plates 105 and 106 toward the fixing roller 101. Therefore, the pressure force of the nip portion cannot be made into a desired pressure force. So, there has occurred the necessity of elevating the end portion 103a on this side of the tension roller 103 to thereby provide a desired pressure force.
That is, in the conventional construction, the deviation control driving portion 115 is discrete from the pressure unit 100B and therefore, when one of the operation of moving the pressure belt toward and away from the fixing roller and the operation of displacing the tension roller 103 is performed, the other operation has been affected.
So, as shown in FIG. 14A of the accompanying drawings, it is conceivable that when the nip portion is released from pressure, the direction in which the inclination of the tension roller 103 is changed relative to the direction indicated by the arrow H in which the tension roller 103 is moved, for the purpose of deviation control, be made orthogonal to the direction indicated by the arrow I. In this case, however, there arises another problem that by the inclination of the tension roller 103 being changed, a tension force applied to the pressure belt 104 is greatly changed.
Also, when as shown in FIG. 14B of the accompanying drawings, the direction in which the inclination of the tension roller 103 is changed (the direction indicated by the arrow J) and the direction in which the tension roller 103 is moved are the same direction indicated by the arrow K, there arises another problem that the deviation control arm 114 becomes a hindrance to the release of pressure during the release of the pressure force.
Also, in both of these methods, there has been the problem that when one of the operation of moving the pressure belt toward and away from the fixing roller and the operation of displacing the tension roller 103 is performed, the other operation is affected.